Cons: Not fully mechanical, only offers MX red switches, can't access in UEFI or to get into UEFI

Just not worth the money. Honestly, the K90 is uncomfortable to type on, and only offers MX Red switches. For 139$, I would expect at least some options. Sadly, Corsair doesn't give us any options and only offers red switches. The excuse is that it's best for gaming. Well, actually the best switches are all in personal preference. Some people like browns over reds for gaming, typing, etc.
On my MSI Z77A-GD55, I can't use the K90 to access the UEFI. My G110 works fine, but the K90 doesn't register anything. So in order to get into my UEFI, I have to go get my G110. That's great... $139 is a hefty price tag, and Corsair's K90 doesn't live up to it.

Cons: Not Fully Mechanical, Some of the LED's seem to die easily, dealbreaking firmware issue

First Let me say that the people over at Corsair did a great job designing this keyboard, however looks can't solve the issues. I would have listed all the cons, but they sort of need an explanation along with them so here they are:

It's not fully mechanical which is sort of annoying but not a dealbreaker as I probably won't be using the Fkeys too often anyway.
On the third day of use I noticed that two of the LED's had died
The Keys are not consistent. What I mean by this is that the some of the keys that are mechanical almost feel different than the others (require more force to be pushed down)

The dealbreaker: This Keyboard will not let you into the Bios depending on your board. The issue is apparently Corsair firmware, and they have avoided the problem on their forums, saying it it a motherboard specific issue, despite being triggered immediately after their firmware update.

You shouldn't have to pay $130 for a keyboard that feels like it still needs quite a bit of work

The Corsair Vengeance K90 is a premium performance mechanical keyboard designed for superior Massively Multiplayer Online (MMO) and Real Time Strategy (RTS) gaming. It features Cherry MX Red mechanical key switches which are of the best of the best key switches and coveted by gamers. They are defined by their low-operating force (45g) and smooth, linear key switch design so you can get quicker double and triple clicks… without the annoying click of a typist's keyboard. 18 dedicated, customizable macro keys provide easy access to your most frequently used macros, presets and key combinations. Full Key Matrix Anti-Ghosting and 20 Key Roll-over on USB detects and reports any simultaneous combination of key-presses with each key press being reported independently. Professional grade brushed aluminum chassis with laser etched, backlit keys provides a comfortable, yet rugged design with outstanding illumination.